1. Field
The invention disclosed and claimed herein generally pertains to a method or procedure wherein heat is used to attach electronic components to a printed circuit board (PCB), or is used to detach components from a PCB. More particularly, the invention pertains to a method of the above type, wherein the thermal mass of one or more components on the PCB is substantially greater than the thermal mass of other components thereon.
2. Description of the Related Art
Surface mount technology (SMT) is a procedure for constructing circuits wherein each electronic component is directly mounted onto the surface of a PCB. Typically, components are attached by applying a solder paste to the PCB, and placing the components within the solder paste. The components are thereby temporarily attached to their respective contact pads of the PCB, by the adherence of the paste. This assembly is then passed through a high temperature oven such as a convection reflow oven, which selectively applies heat to the assembly as specified by a thermal profile or reflow profile. This process causes the solder to melt and to then resolidify, thereby forming intermetallic connections between each of the electronic components and its contact pads on the PCB. Generally, the reflow profile includes several phases or segments. The completed assembly is referred to as a PCB assembly or PCBA.
Notwithstanding the benefits which they provide, currently available SMT procedures have certain drawbacks or disadvantages. For example, it is very common for a PCB assembly to include a component that has a thermal mass which is much greater than the thermal mass of other components of the assembly. As an example of this, an assembly component could be an integrated circuit that comprises a large ball grid array (BGA) package that has a high thermal mass. Accordingly, this component would require a correspondingly large amount of thermal energy during a preheat phase of the thermal profile. However, this amount of thermal energy could cause a second component of the PCB assembly, which has a much smaller thermal mass, to overheat. On the other hand, if the thermal energy supplied during preheat was selected to accommodate the second component, the BGA component could be insufficiently heated for a solder reflow phase of the profile. Generally, because a single pass through the oven is used for all the components of the PCBA, it may be difficult to provide a single reflow profile that is suitable to establish a proper SMT interconnection for each individual component of the PCBA. This in turn can affect the quality and reliability of the PCB assembly.
Problems of the above type can also be encountered, if a PCB assembly having components with very different thermal masses is passed through a vapor phase reflow oven, rather than a convection reflow oven. Moreover, if the assembly has a BGA component with a large thermal mass that must be removed for rework, application of sufficient heat to remove the BGA could overheat adjacent and/or bottom side mirrored components that have a much lower thermal mass, or could cause solder reflow of another BGA component on the same PCBA card.